The Small Computer System Interface (SCSI) is a standard an communications interface having defined electrical and logical to protocol specifications. SCSI allows the interconnection of up to 16 devices on a bus, having differing priorities for access and control of the bus. Typically, a host, which may comprise a computer, such as an IBM RS/6000 or an IBM PC (via a host adapter card), is given the highest priority, limited to one device port, called an initiator and identified by a signal "7", while the peripheral devices are given the remaining lower priorities, called targets and identified by signals "0"-"6" and "8"-"15". The computer uses the bus to interconnect to several target peripheral devices, such as data storage devices, including disk drives, tape drives, etc.
As such, SCSI has become the industry standard for many workstations and their peripherals. However, it is often desirable to allow multiple users and therefore multiple hosts to access common data as stored in peripheral data storage, for example, as a common server system. In one example, such server systems employ a peripheral data storage subsystem, called RAID (Redundant Array of Inexpensive Disks), which provides high speed and fault tolerant access to data stored on a plurality of disk drives. Specifically, the data is stored in a redundant fashion on a plurality of disk drives, so that, should one disk drive fail, the data may still be accessed. In many instances, the disk drives may be accessed in parallel, or may have a solid state memory allowing accumulation of the data, allowing high speed access to the data.
The limitation of the SCSI to a single highest priority initiator device effectively prohibits equal opportunity for access to the common data. Instead, any additional hosts must have lower priorities, and therefore will always be superseded by higher priority hosts. In the case of high rates of access by higher priority hosts, the lower priority hosts may have difficulty gaining access to the common data.